In the absence of a full background model, which cannot be constructed as the origin of the small-S2 background in the detector cannot be reliably quantified, we assume that every event passing the analysis cuts could be due to a DM interaction. The analysis employs the optimum interval method [1]} and will therefore always lead to an exclusion limit. The optimum S2 interval varies with WIMP mass, but
in all cases in this analysis, it contains a minimum of 1000 events
passing all cuts.
The low-mass WIMP result for this 30 kg \(\times \)  yr XENON100 exposure is based on all events remaining in the 80-1000 PE interval (0.7-9.1 keV), the NR acceptance of Fig. REF , and is shown in Fig. REF . At a WIMP mass of 6 GeV/\(c^2\) , XENON100 excludes spin-independent WIMP-nucleon interaction cross sections of \(1.4 \times 10^{-41}\)  cm\(^2\)  at 90% confidence level. The moderate improvement upon the XENON10 low-mass result [2]}, despite the much larger exposure, is due to the significantly higher background from photo-ionization events, which is enhanced by the presence of larger metal surfaces inside the TPC.
The new result challenges a standard WIMP interpretation of the DAMA/LIBRA modulation signal, excludes large fractions of the CDMS-II (Si) preferred region and improves the result of the previous XENON100 result [3]} below \(\sim \) 7.4 GeV/\(c^2\) . We improve the LUX [4]} (SuperCDMS [5]}) results below \(\sim \) 3.7 (5.3) GeV/\(c^2\) .
